The manufacturers of semiconductor devices often demand phosphorus doped single crystal material having a very tight tolerance for average resistivity with a distribution of the dopant as homogeneous as possible. The manufacturers of high-blocking thyristors are interested in such materials because it effects an optimization of the device properties. In contrast to conventional doping methods, it is possible to accomplish the desired doping conditions by means of neutron irradiation. The exact resistivity value can be obtained within an extremely narrow limit and the material is practically free from the macroscopic and the microscopic variations of the electrical resistivity observed in conventionally doped semiconductor materials.
Neutron transmutation doping is the nuclear conversion of semiconductor materials atoms into dopant, i.e., silicon atoms into phosphorus dopant atoms by exposing undoped silicon crystals to a suitable flux of thermal neutrons in the core of a nuclear reactor. The nuclear reaction involved is: EQU .sup.30 Si + n.sub.th .fwdarw. .sup.31 Si + .gamma. .fwdarw. .sup.31 P + .beta..sup.- (t.sub.1/2 =2.6 hrs)
The advantage of this technique is the chance to fabricate N-doped silicon of extreme homogenity which is impossible to realize by conventional doping methods.
Most commercial uses of semiconductor materials require single crystals of very accurately controlled resistivity. In certain instances, this requirement arises from the nature of the semiconductor devices, i.e., the avoidance of marked resistivity variation affecting electrical characteristics. In others, when the device does not give rise to this requirement, characteristics of the starting material may determine the position and nature of junctions and gradients produced during manufacture, so that initially uniform properties in the starting material are necessary.
In most semiconductor materials, constant resistivity is not easily obtained. The distribution coefficients for most significant impurities in, for example, silicon and the increased activity of silicon at its high melting point complicates the problem. Uniform P-type silicon has been produced by either of two methods. Using boron with a distribution coefficient of approximately 0.9, uniform crystals have been produced by crystal pulling. Using the float zone technique with aluminum, having a distribution coefficient of 0.004 P-type silicon of a high degree of uniformity has been produced by zone leveling.
The preparation of uniform resistivity N-type silicon has been a more difficult problem. The usual doping impurities of group V of the periodic table do not lend themselves to use in conventional processing techniques. Certain of these impurities, such as antimony, are sufficiently volatile at the melting point of silicon that it is difficult to control their concentration during the usual crystal growing procedures. The less volatile group V impurities, phosphorus and arsenic, have distribution coefficients which are two small to permit their use in crystal pulling or their use in zone leveling procedures.
Phosphorus has been the group V element preferred for the doping of semiconductor grade silicon to give "N" type electron conductivity, especially for high resistivity materials for power devices. Since the segregation coefficient of phosphorus is relatively low (0.35), a non-planar feeezing interface during the growth of mono-crystals by the float zone process will cause a non-uniform radial distribution of phosphorus in the crystal. Since the center freezes last it will contain more dopant than the edge. Minimizing this non-uniformity, the consequence of which is usually referred to as radial resistivity gradients, has been a goal of research in the zone-refining process. In the past few years, development of the spreading resistance probe method for measuring resistivity has revealed that numerous micro non-homogenities exist in addition to the macro variations revealed by radial gradient measurements. It is generally agreed that the variations in dopant concentration in the crystal lattice play a major role in limiting the performance obtainable in high power silicon devices and in reducing the yield in device manufacturing process.
The crystal damage which is produced during neutron bombardment can be classified by the different types of neutron generation mechanisms. Depending upon the specific reactor performances, a different annealing behavior can be observed. This dependence is correlated with the amount of fast neutrons and can be explained by the increased number of high energetic collisions participating.
From the view of large scale production, various aspects have to be considered like limitations due to residual radioactivity, reactor performances, irradiation capacity, requirements of the starting material, and annealing treatment. It being understood that exposure of a semiconductor crystal to a neutron flux and other radiation within a reactor may produce damage in the crystalline material in the form of a disorder of a crystalline irregularity on a common scale, it will, however, be appreciated that such radiation-produced defects can be cured through annealing by appropriate heating for a specified time length at temperatures within the range of from about 500.degree.-600.degree. C. to higher temperatures below the melt temperature of the semiconductor crystalline material. The annealing has no effect with respect to neutron transmutation-produced nuclides, but results in the removal of radiation damage defects through restoration of crystal symmetry and order. This restoration procedure restores the electrical resistivity to the level corresponding to the dopant-content i.e., in the case of silicon to the corresponding level phosphorus content.
Annealing of the neutron transmutation semiconductor material restores resistivity; however, the minority carrier lifetime is reduced by a significant amount. Even though existing annealing techniques allow restoring of a crystal lattice radiation defect to the extent that the impurity versus resistivity relation is not distinguishable from that of melt doped silicon, i.e., no difference exists between application of neutron and melt-doped silicon apart from the significantly improved doping uniformity, a significant drop in minority carrier lifetime is observed in the case of neutron doping.